Key Properties

• Draw RGB: Enables or disables the graphical object.

• Draw Key: Enables or disables the key signal.

• Automatic Alpha: Calculates the alpha value automatically when set to On. Takes the alpha value entered when set to Off. This is normally done to obtain some level of transparency so that, for example, video background is visible through the key object.

• Alpha as Key only: Takes the alpha value of the object and renders the key with this alpha instead of the alpha value of the key function attached. The object itself is rendered opaque. This is usually used to render transparent objects for keyed graphics. The color mixing of the object in the scene and the, for example, video background is done by the external keyer so the object must be rendered non transparent.

• Render Mode: Sets the mode in which the alpha values of the key item to be rendered should be mixed with the alpha values already existing in the frame buffer.

  Note: Be aware that with the subtractive and the rev-subtractive, mode the order in which the objects are being rendered then is crucial. Make sure the z-sort is correct.

  The mixing is done on a per pixel basis (in the formulas the range of the alpha values is from 0-1, instead of 0-100 as in the value field).
  If A is the current source alpha value being rendered now and FA is the alpha of the frame buffer contents (target), the different modes can be explained as following:

  • Add: (A + FA) The source alpha gets added to the target alpha. As the formula shows it is an addition, so the end result is always a higher alpha value than the initial frame content. If the frame alpha has a high value, you might experience that the addition gives a total value above 1. This is clamped at 1, which is opaque.

  • Blend: (A+(1-A)*FA) The new alpha value gets created as a weighted average of the source and the target. If we as an example say that the key being rendered has alpha 1 and the frame alpha has 0.5, the formula is: 1+(1-0.5)*0.5 = 0.75.

  • Subtract: (FA-A) The new alpha value is the result of the incoming alpha being subtracted from the alpha in the frame buffer and the result is written back into the frame buffer.

  • Rev-Subtract: (A-FA) This is a reversed version of subtractive. The alpha in the frame buffer gets subtracted from the incoming alpha and the result is written back into the frame buffer.

  • Max: Uses the maximum of all key signals.

  • Combine with BG chroma key: Combines the alpha of the Container with the alpha of the background, before blending the foreground with the background (active in a virtual sports render sequence only) when set to On.

  • Depth information only: Occludes depth only for rendering with live video. Containers which are marked for Depth Only are rendered before Containers which are marked for chroma or linear keying. This could be useful for sports, where real objects should occlude virtual items of the scene. This setting requires the Render Sequence to be set to Arena in the scene’s Global Settings Panel.

To Add Key

1. Open a Scene.

2. Add an object.

3. Add Key plug-in.

   

4. Click the Key button on the left side of the Scene Editor.

   

See Also

• Global Settings Panel